Long periods without sunshine can play a role in a surprising variety of physical and mental disorders

The year's darkest day is drawing near. December 21 marks the winter solstice in the Northern Hemisphere, the shortest day and longest night of 2014. Thanks to artificial illumination, winter's lack of sunlight doesn't change our lifestyles quite as much as it did in the past. But our bodies definitely take notice—scientists are discovering that prolonged darkness can play a role in disorders from depression to diabetes. The consensus seems to be that sunlight is essential to humans, provided we can get the right dose on a regular basis.

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Most people know that too much exposure to ultraviolet (UV) rays from the sun can cause cataracts and skin cancer. The World Health Organization estimates that between 2 and 3 million rarely lethal non-melanoma skin cancers occur globally each year, along with 132,000 far more serious cases of melanoma skin cancers.

Protecting the skin with lotions and clothing and avoiding too much time in searing sun can drastically reduce the odds of developing of skin cancer. But avoiding sunshine altogether isn't a great idea, because the light can produce a plethora of positive health impacts. For starters, UV rays in sunlight trigger a photosynthetic process in the skin that produces vitamin D. This vitamin's active form may help to regulate more than 1,000 genes, which in turn govern most of the body's tissues. Vitamin D is also crucial for bone health and for keeping the immune system going strong.

Insufficient amounts of vitamin D can trigger ailments such as rickets, a weakening of the bones that can cause skeletal deformities and dental issues. Vitamin D deficiency can also lead to the cutaneous form of tuberculosis, which causes painful lesions. One recent study even associated deficiency of the vitamin with an increased risk of dementia and Alzheimer's disease in older people. Severely deficient patients were more than twice as likely to develop these ailments, the study found.

Light treatments using both natural light and artificial lamps have some history as preventative measures and even as treatments for diseases linked to vitamin D deficiency. And moderate exposure to natural light usually produces an appropriate amount of vitamin D, although the exact amount varies greatly with climate, skin pigment and other factors. Dietary supplements and oily foods can help to make up the difference.

“The most interesting recent development to me is the [number of] probable other beneficial effects of a bit of sun exposure or time outdoors,” says epidemiologist Robyn Lucas at the Australian National University. Lucas was lead author of a World Health Organization study about the global health burden due to UV exposure. “There are recent studies showing beneficial effects on blood pressure, development of obesity and modulation of immune function to be less autoreactive, so lower incidence of autoimmune disorders like multiple sclerosis.”

One recent study in the Journal of Investigative Dermatology suggested that exposing skin to sunlight can reduce the risk of stroke or heart attack, possibly because it alters levels of nitric oxide in human skin and blood. When the sun shines, small amounts of this messenger molecule are transferred from the skin to the circulatory system, dilating blood vessels and lowering blood pressure, the research posits. And in preliminary work in laboratory mice, nitric oxide released by UV exposure seemed to help curb weight gain and lower the risk of diabetes—although the researchers caution that they can't yet be sure whether the effects translate to people.

Mental health can also take a hit during the year's darkest days. Seasonal affective disorder (SAD) is a subtype of depression that involves many of the same symptoms, including loss of energy, lack of interest in enjoyable activities, oversleeping and feelings of hopelessness. Decreased sunlight can cause drops in your body's production of serotonin, a brain chemical that helps to determine mood. Lack of light can also alter the brain's balance of melatonin, a chemical produced during the hours of darkness that helps to govern sleep patterns and mood.

Some studies have suggested that SAD sufferers adjust their nightly melatonin levels seasonally, producing it for longer durations during the dark winter, while non-suffering people don't change. Those findings may lend credence to other research suggesting that SAD is an evolutionary holdover from an era when humans were adapting to changes in seasonal food availability. Like their ancestors, (and many modern animals) SAD sufferers may be genetically driven to eat more and conserve energy during lean periods like winter.

Unlike other forms of major depression, SAD often fades when the sun returns. And while it shares conventional depression treatments like medications or psychotherapy, SAD can also be countered by phototherapy: flooding the body with an artificial version of sunlight. The idea is that dwindling natural light alters the body's circadian rhythms, contributing to SAD. Mimicking that light can first fool the eyes and then the brain into restoring normal rhythms long before they'd naturally ease with the return of spring. Within just a few days, in some cases, phototherapy sessions appear to recalibrate levels of the brain chemicals impacted by a lack of natural light.

“We evolved getting a bit of sun exposure,” says Lucas, and the evidence is suggesting that we all need some rays to stay healthy. That will become incrementally easier to do after December 21, because each subsequent sunrise promises a bit more light to enjoy.